The best reagent for converting 2-phenylpropanamide into 2-phenylpropanamine is:

LiAlH₄ in ether

The correct answer is option 4. LiAlH₄ in ether

The reaction of converting 2-phenylpropanamide into 2-phenylpropanamine is shown below: -

As we can see that the amide group of reactant is changed into amine group or we can say that the carbonyl part of amide group is converted into amine group without any changes in the benzene ring or any other part of it. So we require a reagent that does the same when reacted with an amide functional group.

1. Excess \(H_2\): When hydrogen gas reacts with an unsaturated compound, it reduces it and changes it into a saturated compound. It could be used in this reaction but this reagent also reduces the benzene ring, hence this is not the best reagent for the above conversion.

2. \(Br_2\)​ in aqueous \(NaOH\): This can be used to convert an amide group to amine group which is known as Hoffmann rearrangement. But it also requires heat for the decarboxylation step. Hence it is not the best reagent as well.

3. Hydrogen iodine in the presence of red phosphorus: this combination forms \(PI_3\) which leaves only hydrogen atoms that completely replaces alcohol group from any compound or in simple words, it reduces alcohol group. Therefore, is not the best reagent for this conversion.

4. \(LiAlH_4\)​ in ether: This reagent is especially used in the reduction of carbonyl groups without affecting the double bonds of the benzene ring. So it can easily convert the amide part of 2-phenylpropanamide to amine. Hence this is the best reagent for this conversion.